The simplified message service interface (SMSI) protocol, also known as the simplified message desk interface (SMDI) protocol, is a standard protocol for data communications between communication system switches, such as telephony central office switches and PBXs, and adjunct processors, such as voice message systems. Unfortunately, the protocol is not robust: it does not provide a mechanism, such as an end-to-end "handshake", for checking the continued viability of the data link.
Data communications using this protocol are typically conducted over conventional RS-232 data links. The RS-232 physical layer protocol does provide a means for checking the connectivity of a data link by looping-around one of the protocol's leads. However, this mechanism only serves to check the physical, electrical, connectivity of the link, and provides no check on the proper transmission and receipt of, and response to, the information content of a sent communication.